There is growing demand for improved efficient management of energy consumption in jet engines and automobiles and the global reduction of undesirable emission of hydrocarbons and other combustion by-products, such as Nitrogen oxide (NOx) and Carbon Monoxide (CO). Semiconductor-based sensors and electronics targeted for insertion in high temperature, extreme vibration, and corrosive media must satisfy a set of minimum reliability criteria before becoming acceptable for operational use. In addition, it is crucial to validate the Computational Fluid Dynamics (CFD) codes generated for flow fields and turbulence conditions inside engines. Non-validation of these codes renders them untrustworthy for use in future engine designs.
Devices (sensors and electronics) capable of functioning in these harsh environments need the appropriate package to sustain stable and reliable operation during the life cycle of these devices. Package reliability problems have largely contributed to prevent the practical application of these devices. The temperature of the instrumentation environment is typically greater than 300 degrees Celsius (° C.). Therefore, devices must survive and operate reliably beyond that temperature. This is very challenging since conventional semiconductor electronic and sensing devices are limited to operating in temperatures less than 300° C. due to the limitations imposed by the material properties and packaging. Silicon carbide (SiC)-based electronics and sensors have been demonstrated to operate at temperatures greater than 600° C., thereby offering the promise of direct insertion into such high temperature environments.
However, the lack of the device packaging methodologies appropriate for such harsh environments has affected the operational reliability and survivability of these devices. Economically, reliability problems at high temperature due to poor packaging have discouraged global application and large-scale commercialization. As a result, reliability problems have contributed to delay the much-anticipated early introduction of SiC devices into high temperature environments.
Generally, the primary methods of bonding the SiC sensor and the cover member are either by electrostatic bonding or by direct bonding using a glass flits. In the cover member, an aperture is drilled to serve as an escape path for gases during the curing of the bonding glass frits. The aperture is eventually sealed to provide hermetic sealing. The disadvantages of such a method include that the use of an electrostatic bonding method makes very weak bond strength between the SiC sensor and the SiC cover member. This may lead to debonding during thermal cycling, thereby rendering the device useless.
The application of glass fits as the adhesion material between the SiC cover member and the SiC sensor wafer also makes necessary the creation of an aperture as an escape path for outgassing during glass bonding. Since the aperture will have to be sealed later in order to maintain the desired hermetic reference cavity, creation of apertures increases the risk of the sealant slipping into the reference cavity. Because SiC has a very low oxidation rate, the SiC cover member must be heavily oxidized in order to create a thick oxide to envelope the SiC cover member to prevent electrical conduction during operation at high temperature. The oxidation process could take as long as twenty-four hours and may break down during a current or voltage surge. There are many components coupled together, which also raises problems of thermomechanically-induced stress on the sensor, thereby gradually degrading sensor performance.
However, the solutions associated with the above-mentioned problems have the operational limitation that they do not easily lend themselves for multifunctional applications.